Various methods have been used to measure the wall thickness of pipe as it is being extruded. Often a rotary type scanner is used which permits inspection of the circumference of the pipe along a generally helical path, with the pitch or axial spacing between adjacent portions of the helical inspection path being determined by the relative rotational speed of a sensing probe used to inspect the pipe as well as the relative axial speed of the pipe.
A rotary scanner generally includes a frame having a rotatable ring. Extruded pipe is directed through the ring and ideally the pipe is centered within the ring. A sensing probe is connected to the rotatable ring and is directed generally radially toward the center of the ring and pipe. Rotation of the ring enables the probe to scan a full 360.degree. around the pipe.
Typically, the scanner is located as close as possible to the pipe extrusion outlet in order to minimize the length of lost pipe after a flaw has been detected by the probe. However, a first cooling station is generally placed between the extruder and the scanner to firm up the pipe wall prior to its measurement by the probe. Even then, the flowing pipe is still hot and soft and a second cooling station may be necessary after the pipe has been measured. In this system, the first and second cooling stations can act as supports for the pipe as it passes through the scanner.
As the pipe passes through the rotary scanner, its wall thickness is continuously measured by the sensing probe. The information obtained by the probe is used to provide a thickness profile of the external surface of the pipe. Deviations from standard thickness may be corrected immediately, for example, by adjusting die bolts on the extruder.
One type of probe particularly adapted for use with a rotary scanner is a nucleonic gamma gauge which operates on the principle of Compton photon backscatter (commonly known as gamma backscatter). The sensing probe holds a radioactive isotope that emits low-energy gamma rays (photons) which are collimated and beamed at the inspected material. The gamma rays are then scattered back toward a detector portion of the probe in direct proportion to the thickness of the material in front of the probe. An efficient scintillation crystal/photomultiplier detector converts the backscattered photons to an electrical signal, which in turn can be related to weight-per-unit area or thickness (if material density is constant).
Because gamma probes can measure thickness from just one side of the material, they are ideally suited for measuring wall thickness of pipe and tubing. To achieve maximum effectiveness, however, the gamma probe (and collimated beam) should be maintained normal to, and in contact with, the surface to be measured. This is particularly difficult to do with freshly extruded pipe, which is still quite flexible and easily deflected, even after it has passed through the first cooling station.
For example, because of its flexibility and flowing speed, and because of the spacing between the supports of the first and second cooling stations, the extruded pipe tends to wobble as it passes through the rotary scanner. This wobbling can cause a radial displacement of the pipe relative to the scanner ring, i.e., the pipe is not stable in the ring and changes location during probe rotation. Because of this wobbling, two problems occur with respect to accurate measurement of wall thickness by the sensing probe. First, the probe may lose contact with the wobbling pipe and, second, the probe will not remain radially directed toward the center of the pipe and thus will lose its perpendicular relationship to the exterior surface of the pipe.
One method used to correct the first problem has included the use of a spring-loaded probe holder which exerted a force on the probe, pushing it toward the exterior surface of the pipe. The spring force, however, occasionally provoked deformation or scratching of the soft pipe and corresponding inaccurate readings from the probe were obtained. Furthermore, it was often necessary to use an excessively high spring force to overcome the probe weight when the probe was located in the six o'clock position of the scanner (i.e., the probe pointed vertically upward and inward). This, however, resulted in the probe applying a contact force of greater than twice the probe weight when the probe was in the twelve o'clock position (i.e., pointed vertically downward and inward).
One method used to correct the problem of maintaining the probe directed toward the center of the pipe included the use of a pair of tangent V arms or a pair of rollers secured to a rotatable probe support. The arms or rollers would contact the exterior surface of the pipe at two points around the circumference of the pipe causing the probe support and probe to rotate and follow the pipe as it wobbled. However, this arrangement could not efficiently cover a large range of pipe diameters and did not insure that the probe would remain in nondestructive contact with the pipe.
Accordingly, a need has arisen for a probe holder that maintains a probe in contact with an external surface of a pipe to be measured, such that the contact force between the probe and pipe is maintained at a small and constant value at any circumferential position around the pipe. It is also desirable that such a holder point the probe toward the center of the pipe regardless of the wobbling effect acting on the pipe as it passes through the scanner ring. Such a probe holder should also be fully counterbalanced so as not to exert any excessive force or cause damage to the pipe being measured.